This invention involves the operation of a piston in a cylinder under conditions such that liquid lubrication cannot be used successfully. Such conditions arrive, for example, in devices used for compressing oxygen, where it is highly hazardous to allow compressed oxygen and organic materials to come into contact. They also arise in hot gas engines.
A hot gas engine consists of a closed interconnected assembly of spaces or chambers, with one such space heated and another cooled, a mechanism for transferring gas alternately between hot and cold spaces thus producing variations in pressure, and a cylinder and piston for converting such pressure variations into useful work. Hot gas engines have been studied for many years and have taken various forms.
In the construction of hot gas engines it has been usual to avoid the problem of operating a piston in a hot cylinder by locating the piston or pistons in a cool location where it can be lubricated by the use of conventional organic lubricants. This is an unsatisfactory solution for two reasons. Thus placing the piston forces compromises on design which may adversely affect efficiency or output per unit of weight. Second it involves the probability that oil droplets or vapor arriving at heated spaces of the engine will there decompose to form coatings which will severely limit engine life. This is especially serious for very high temperatures, now practicable with modern materials, are necessary if high efficiency is to be attained. This invention is concerned therefore with omitting liquid lubrication entirely, and locating a piston in a cylinder which will become hot during engine operation, although not as hot as the location of maximum temperature.
This problem has been approached by using piston rings capable of operating successfully at the maximum temperatures to which they will be subjected. If these temperatures are moderate, rings of organic material, commonly called plastics, may be used. At higher temperatures it is possible to use rings made of inorganic materials. Such material, for example consisting of sintered powdered calcium fluoride with its pores filled with lead monoxide, is often referred to as solid lubricants.
There are two objections to that solution of the problem. First, the coefficient of friction of any material operating against a dry surface is higher than one operating against a surface which is liquid lubricated, and that involves higher losses. In this connection it should be noted that, for large output per pound of a hot gas engine, high gas pressures are indicated, and that results in rings, as ordinarily designed, being pressed against the cylinder walls with high pressures per unit of area. Second, at these high unit pressures, ring wear is increased, which tends to limit engine life before overhaul.